Biomass, especially lignocellulosic biomass, is a valuable resource for the production of (bio)fuels, chemicals, performance products and energy. Lignocellulose is the most abundant renewable biomass available on land, and therefore relatively cheap. It comprises mainly cellulose, hemicellulose and lignin. Many research efforts have been devoted to the development of processes for the cost-effective conversion of biomass, especially lignocellulosic biomass, to valuable compounds. An example thereof is the conversion of cellulose to glucose, which in turn may serve e.g. as a precursor for ‘second-generation’ bioethanol (e.g. by fermentation of glucose), and is thus suitable for the production of biofuels.
The main structural components of biomass are cellulose (a glucan), hemi-cellulose and lignin. The two major types of hemicellulose are xylans and (gluco)-mannans. Xylans have xylose (C5 sugar) backbones, sometimes substituted with arabinose or glucuronic acid side groups, and are predominant in hardwood and grasses, while (gluco)mannans have backbones with a glucose:mannose (both C6 sugars) ratio of about 1:3, sometimes substituted with galactose side groups, and are predominant in softwood. Minor hemicellulose types include xyloglucans and arabino-galactans. Hemicelluloses may be chemically linked to lignin. Table 1 below gives approximate compositions of some biomass types.
TABLE 1Compositions of the structural components of somebiomass types (in wt % based on dry weight)other poly-glucanmannanxylansaccharidesligninsoftwood35-4015-20 5-10 3-1025-32hardwood40-501-415-302-522-30grasses, straws33-400-220-273-820-32
The so-called organosolv process can be used to treat biomass (pretreatment), in order to make cellulose polymers better accessible for hydrolytic enzymes converting cellulose to glucose, or for pulping or fractionating of the biomass. Without pre-treatment, the cellulose within lignocellulosic biomass is poorly accessible for the hydrolytic enzymes, as it is shielded by other structural components in the biomass, such as lignin and hemicelluloses. Conventional organosolv involves high-temperature treatment (typically between 180 and 220° C.) of the biomass with a (water-miscible) organic solvent (e.g. ethanol) and optionally an (acidic) catalyst. During organosolv fractionation, the lignocellulose biomass is fractionated into a cellulose-enriched solid product stream (pulp) and a liquid product stream (liquor) comprising dissolved lignin and hemicellulose derivatives.
The hemicelluloses present in the lignocellulosic biomass are relatively unstable and break down during organosolv. Hemicellulose is first hydrolysed to sugar monomers (C5 and/or C6 sugars), which may subsequently dehydrate to furans such as furfural and/or react further to other compounds (including xylosides and condensation products with lignin (“pseudo-lignin”)). Most of these latter compounds are considered less valuable, with a smaller demand for in the market, than hemicellulose itself or the products directly obtained from it such as monomeric sugars (mainly xylose, mannose and glucose). Degradation products may be part of the cellulose stream and/or the lignin stream, which are produced by the organosolv process, thereby reducing their purity and the efficiency of further treatment of these streams to produce valuable end-products, such as ethanol. In addition, potentially valuable compounds that can be derived from the hemicellulose (e.g. monomeric sugars and furfural) get lost, thus reducing the effectiveness of the conversion of biomass into valuable components.
Also, the cellulose-enriched product stream obtained from the organosolv process comprises impurities. Although organosolv treatment separates large parts of lignin and hemicellulose from the cellulose pulp, the cellulosic pulp typically still comprises significant amounts of lignin, as well as pseudo-lignins. The latter may be formed during organosolv fractionation by reaction of lignin with e.g. proteins, other extractives and/or furans such as furfural. These impurities hamper the enzymatic hydrolysis of cellulose to glucose, which is to date still not feasible on a commercial scale, since it cannot compete yet with glucose produced from first generation biomass sources (starch, sucrose etc.), in view of the high costs of the pretreatment step and required amounts of enzyme. Alternatives to enzymatic hydrolysis of cellulose, e.g. concentrated acid treatment, are undesirable for environmental reasons, corrosion of equipment and associated costs, and they are typically less selective towards glucose because of enhanced sugar degradation reactions. Hence, one of the challenges of current research is to find means to enhance the efficiency of pretreatment of biomass and (simultaneously) improve the enzymatic hydrolysis of cellulose, in order to allow application on an industrial scale.
WO 2007/120210 describes organosolv treatment of biomass at about 120-220° C., at a pH of less than about 4, and with ethanol as preferred solvent. The organosolv reaction is performed at 170° C. and subsequent separation of the solids from the liquids by filtration is performed at 130° C. WO 2012/000093 and WO 2011/097720 describe organosolv treatment of biomass at 130-170° C., at a pH of 1.5-2.5, and with ethanol comprising 1.5-2.5 wt % acid as preferred solvent. Da Silva Perez and Curvelo (Open Agriculture Journal 2010, 4, 145-152) studied the kinetics of acetone-water delignification of Eucalyptus urograndis at temperatures ranging from 145 to 195° C. and found that at the lower temperatures the least efficient delignification occurred. Huijgen et al. (Ind. Eng. Chem. Res. 2010, 49, 10132-10140) describe acetone-based organosolv of wheat straw at temperatures ranging from 160 to 220° C., in the absence of an acid.